1. Field of the Invention
The present invention is related to circuit design systems and more particularly to computer aided design (CAD) systems for designing integrated circuits (ICs) operating over a wide frequency band and with circuit interconnects that might experience transmission line effects.
2. Background Description
Sets of software programs are well known in the art as computer aided design (CAD) tools. Typical CAD tools include automatic layout tools, timing analysis tools, logic synthesis tools, and so forth. In addition, a CAD tool may include electromagnetic (EM) field solvers (also known as parameter extractors) and circuit simulators for circuit conductor (interconnect) analysis and design. Integrated circuit (IC) designers have found CAD tools invaluable for designing, analyzing and verifying complex ICs and IC chip designs, such as microprocessors, microcontrollers, communications circuits and the like.
Providing an acceptable wide-band conductor description requires an accurate description of the particular conductor, e.g., its geometry and construction materials. This information may be available to the semiconductor process technologist but usually is not passed to the circuit designer. Instead, the circuit designer receives a set of ground rules that describes a range of results based on those conductor properties, e.g., capacitance/resistance/inductance per unit length or per unit area. These lump values at best provide rule of thumb design guidance and do not lend themselves to accurate wide-band electrical circuit characterizations of high-performance electrical interconnect structures.
Designers have used EM field solvers to compute conductor electrical properties, such as capacitance and inductance, from conductor geometric and physical specifications. Then, the designer can use the EM field solver results in a circuit simulator to simulate the transient or AC response, i.e., how the particular circuit devices connected by and driving the conductors respond to excitatory input signals. The excitatory input signals can be modeled to include either or both wanted and unwanted input signals, e.g., a driving logical signal such as from another such circuit, and noise signals, such as electromagnetic interference from the neighboring conductors and/or coupled noise from adjacent lines (cross talk).
For circuit signal stability the circuit model must have a valid low frequency or DC response, as well as an acceptable very high frequency transient response, e.g., to 100 GigaHertz (100 GHz) and beyond. This complicates circuit design because in addition to active element models that are accurate over the expected operating frequency range, either the circuit model must somehow accommodate wide-band models for on-chip conductors or different conductor models must be employed at each of a number of points across the frequency spectrum to accommodate the full range of conductor characteristics. Modeling conductors across this range requires a very-high level of expertise in computational models for electromagnetism that even the best IC designers do not normally have. See, e.g., Alina Deutsch et al., “On-Chip Wiring Design Challenges for Gigahertz Operation,” Proceedings of The IEEE, Vol. 89, No. 4, April 2001, pp. 529–555.
Thus, there is a need for electrical circuit conductor descriptions that are valid over a very wide range of operating frequencies and in particular there is a need for such conductor descriptions in circuit design.